Filtering apparatus



March 21, 1933. w. H. T. HOLDEN EILTERING APPARATUS Filed Jan. 6, 1931 Plaza Conectu .Freqqeiacy INVENTOR WEZEZden/ BY ATTORNEY Patented Mar. 21, 1933 UNITED srAT Lauren ES PATENT OFFICE WILLIAM H. T. HOLDEN, OF BROOKLYN, NEW YORK, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK FILTERING arraimrus Application filed January 6, 1931. 'Seria1 No.'506,960.

This invention relates to signalingsystems. and to filters and current-suppresslng sys tems. More particularly, this invention relates tosystems arranged so as to greatly increase the effectiveness of filters or current-.

be considered to provide apparatus'for con-v verting a cheap filter of one type such, for

example, as the band elimination type, into a very effective filter of a different type, such, for example, as the band pass type, and vice versa.

This invention may be broadly considered as comprising two electron tubes'which may form an amplifier having two parallel paths to which currents of the sarnefrequencies are transmitted currents which have equal amwill be nullified in a common circuit connected or otherwise coupled to these transformers. However, currents which do not have equal amplitudes after transmission through the two parallel paths, will not be completely nullified, and their'eflects will be pr'operly registered in the common circuit.

This invention will be bet er understood from the detailed description hereinafter fol-, lowing, when read in connection with the accompanying drawing, in which Fig. 1 represents an embodiment of the invention given merely for thepurpose of illustration, and

transmitted, some of which have smaller am-- plitudes will exhibit equal and opposite effects in the transformers, and these efiects:

Fig. 2 represents a number of curves to ex-. plain the features of the invention. q 7

Referring to Fig. 1 of-the drawing,the reference character 0 designates an input circuit which supplies a pluralityofcurrents 551.;

of a wide variety of frequencies which may. be those forminga very broad band and may include other bands and individual currents. All of these currents are transmitted through a transformer T to afilter designated F which may,'for example, be of the band elimination type. Such a filter will substantially; suppress currents having frequencies between certain well defined limits, while freely transmitting currents of all frequencies with- 6553 out these limits. Between the limits. of the filter F all currents will be attenuatedbya somewhat definiteamount, and the magni+ tude of the attenuationwill depend on the quality or the design of the filter, and this in turn usually depends on the number of sections included in the filter. It is awell known fact that the effectiveness of the filter, i. e., the amount of attenuation it presents to certain undesired frequencies as compared toother desired frequencies, is raised byin creasingthe number of its sections, and con versely a filter having a smaller number of sections will be one of lower effectiveness. The filter F. may be composed of asingle sec tion or may include. merely a few sections, i. e., two or three sections-and thesingle section of the few sections may each be of a type, which includes a piezo-electric crystal.

The currents freely transmitted by the filter F substantially without attenuation and those having frequencies within the limits of this filter transmitted with quite largeand substantially uniform attenuation, are im-. pressed on the grid and filament electrodes of an amplifying tube V through a trans-.' former T and a potentiometer P. A battery B fixesthe bias of the grid electrode of the tube V with respect to its filament. -The potentiometer P may be manipulated to change the gain ofthe currents transmitted through. transformer T The currents. amplified by the tubeV appear in its output circuit, which includes a battery B the primary winding 1 transformers T of a transformer T and the plate and filament electrodes of the tube V The currents transmitted from the input circuit 0 through transformer T also reach a phase correcting network designated N which may be of any well known type and which is employed to equalize the phases of the currents transmitted therethrough with those transmitted through the filter F. This neutralizing network is designed to be especially efficient for currents having frequencies within the limits of the filter F, where in precise phase correction will occur. Thereafter, these currents become impressed on the grid and filament electrodes of another amplifying tube V through a transformer T V The tube V is an amplifier of the same general type as V and is preferably one equalin all of the tube characteristics. Especially should these tubes be equal in their amplifying factors, for both should equally amplify all currents reaching their input circuits.

The currents amplified by the tube V flow through its output circuit, which includes the battery E the primary winding of a transformer T and the plate and filament electrodes of the tube V It is to be noted that the battery B equally'biases the grid electrodes of the tubes V and V that battery B provides the same'positive potentials for the plate electrodes of these tubes and that their filament electrodesare connected in parallel relationship with respect to a common battery B The primary windings of the transformers T and T are so connected that their effects may be continually in opposition. Equal currents of a common frequency impressed upon these windings will have practically no effect in an output circuit 0 which is connected in series with the secondary windings of the and T WVhile transformers do not have the theoretically perfect balance, their balance should, in the practice of this invention, be equal tothe gain of either of the tubes V or V The transformers T and T; have identical characteristics and, therefore, voltages transmitted therethrough which are 180 degrees out of phase with each other in the output circuits of tubes V and V will annul each other, while voltages which are in phase with each other or are out of phase with each other by some angle different from 180 degrees will not annul each other.

Current flowing through the primary windings of the transformers T and T which have equal amplitudes, will oppose each other and their effects will not appear in the output circuit 0 This will be the case with currents of all frequencies outside the limits of the filter F.

Currents of all frequencies within the limits of this filter will, however, have difierent effects upon the primary windings of the transformers T and T These currents will afiect the primary winding of the transformerT more greatly than the primary winding of the transformer T and currents of all of these frequencies will be found in the output circuit 0 Curve 0 of Fig. 2 represents the relation between the frequency of various currents and their gain after amplification by the tube V This curve shows that currents of all frequencies outside the limits of the filter F will be transmitted quite uniformly and at a high gain and that those currents of frequencies within these limits will be substantially uniformlyattenuated. If the tubeV amplifies currents having frequencies outside the limits of the'filter F to a level of, for example, about 24 decibels, it may be assumed that the level of the-currents of frequencies within the limits of the filter F will, for example, be about 7 decibels. This-filter may be considered as one which. causes an attenuation of about 17 decibels at undesired frequencles.

Curve d of 2 represents the level of the energy passing through the phase correcting network N and amplified by the tube'v Here all frequencies are equally amplified, especially because the network N has practically no frequency discrimination. Currents of all frequencies will have substantially the same level and this may be assumed for illustrative purposes to be, for example, about 24 decibels.

Curve 6 of Fig. 2 shows the natureof the currents flowing through the output circuit 0 Practically all currents having frequencies within the'limits of the filter F freely flow through the circuit O and have a level almost equal to the levelof those currents flowing in the output circuit of the tube V Currents of frequencies within the limits of the filter F will be greatly attenuated and their level, in the illustrated case, may be about 24 decibels less than the level of those currents having frequencies within-the limits of the filter.

It will be apparent that a device having the characteristic 6 exhibitsan attenuation for frequencies outside certain limits considerably greater than a device having a characteristic such asc which exhibits an attenuation for frequencies withinthese limits. In the illustrated case this difference in attenuation may, for example, be about 7 decibels. Thus, the efiectiveness ofa device having the characteristic 0 greatly exceeds the one with the characteristic 0. q

'The figures g ven in the illustration are not to be considered as in any way limiting the invention, forthese may be greatly varied in difierent cases without departing from the spirit of the invention and the scope of the appended claims.

It will be seen that the arrangement described herein involves apparatus for addition to an inefiicient filter of poor quality which may be of the band elimination type in order to render the arrangement a high quality filtering apparatus of, for example, the band pass type. The arrangements of this invention in efiect convert a filter of one type, i. e., the band elimination type, into one of a very different type, i. e., the band pass type. If the filter F were of the band pass type, the over-all circuit would have a characteristic of the band elimination filter. This inversion of properties may be avoided, however, if such is desired, but in that event the band elimination filter F shown in Fig. 1 would have to be one having a very flat characteristic for currents of frequencies within the limits of the filter. Such a device is not ordinarily found and must generally be specially designed.

While this invention has been disclosed in certain particular arrangements merely for the purpose of illustration, it will be understood that the general principles of this invention may be applied to other and widely varied organizations without departing from the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. Band pass filtering apparatus comprising a band elimination filter, two amplifying circuits coupled, respectively, to the input and output circuits of the filter, both amplifying circuits having equal gains, and a pair of transformers connected to said circuits and coupled so as to be in opposition.

2. The combination of a source of currents having different frequencies, a band elimination filter, a phase correcting network in parallel with said filter with respect to said course of currents, two equal amplify ing circuits coupled, respectively, to said filter and said network, and a pair of transformers connected to said amplifying circuits and coupled so as to be mutually opposing.

3. The combination of a filter greatly suppressing currents of frequencies Within cer tain limits and freely transmitting currents of frequencies without said limits, and means coupled to said filter for converting the combination into one freely transmitting currents of all frequencies within said limits while greatly suppressing currents of all frequencies without those limits, said means including a phase correcting network.

4. The combination of a band elimination filter, apparatus connected in parallel with with said filter for equalizing the phases of currents transmitted therethrough with those freely passed by the filter, two amplifiers coupled respectively to the filter and the equalizing apparatus, and means for ing a loss equal to the gains of said amplifiers.

In testimony whereof, I have signed my name to this specification this 5th day of January 1931.-

WILLIAM H. T. HOLDEN. 

